The solids content and ratio of soluble to insoluble solids is a major factor affecting the profitability of commercial tomato processing operations. The solids content is also important in determining the flavor and marketability of fresh market tomatoes.
Tomato solids include a water-soluble and a water-insoluble fraction. The insoluble solids in tomato fruit are primarily components of the cell wall and are responsible for the viscosity of processed tomato pulp. The water-soluble fraction contains the hexoses, glucose and fructose, which constitute more than 90% of this fraction. Measurement of the content of these two sugars in a given fruit defines the "soluble solids content" of that fruit for commercial cultivars.
The hexoses in ripened tomato fruit are produced by hydrolysis of sucrose that is transported from the leaves and by hydrolysis of accumulated starch, which is also derived from sucrose transported into the fruit during fruit development. The enzyme which catalyzes the conversion of sucrose to the hexoses glucose and fructose, is beta-fructofuranosidase, commonly called invertase. Plants, including tomato, have at least two invertase activities, a soluble invertase located in the vacuole and an insoluble invertase activity bound to the cell wall.
There are characteristic differences in the distribution of sugars and the activity of invertase in plant tissues and in the fruit at different stages of ripeness. Invertase activity increases in tomato fruit during ripening. There are also differences in the activity profile of invertase and in the solids content among the fruits of different tomato species. For example, the fruit of Lycopersicon pimpinellifolium, which is a wild tomato species, is richer in invertase and expresses it earlier during ripening than the cultivated tomato species, Lycopersicon esculentum.
Tomato growers and processors strive to attain a tomato fruit that reflects the specific balance of soluble solids content and insoluble solids content desired for a particular tomato product. Traditionally, efforts to improve or alter this balance have focussed on the development of hybrid plants by crossing species of cultivated tomatoes with wild tomato species that produce fruit with a higher soluble solids content than the cultivated varieties. The hybrids, however, also tend to possess undesirable traits of the-wild species.
There is a need, therefore, to produce improved versions of cultivated species of tomato, such as L. esculentum, that exhibit desirable traits of the wild species, such as a higher ratio of soluble solids to insoluble solids and a higher level of soluble solids, but that do not also have the undesirable traits of the wild species. It would also be desirable to have the ability to produce cultivated plants that have a selected specific level of soluble solids content and ratio of soluble to insoluble solids content desired for a particular tomato product.
Therefore, it is an object of this invention to provide transgenic tomato plants that express invertase earlier during ripening and express higher levels of invertase during fruit ripening than cultivated non-transgenic plants.
It is also an object of this invention to provide a means for regulating and altering the levels and ratios of soluble to insoluble solids in the fruit of cultivated tomato plants in order to select a specific level of soluble solids content and ratio of soluble to insoluble solids content desired for a particular tomato product.